GEOLOGIC CHARACTERIZATION SUPPORTING ENHANCED OIL RECOVERY PILOT, CITRONELLE OIL FIELD, SOUTHWEST ALABAMA

Citronelle Dome is a giant salt-cored anticline in the eastern Mississippi Interior Salt Basin of southwest Alabama. The dome forms an elliptical structural closure containing multiple opportunities for enhanced oil recovery (EOR) and saline reservoir CO2 sequestration. Citronelle Oil Field, located on the crest of the dome, has produced more than 169 MMbbl of 42-46° API gravity oil from the Lower Cretaceous Donovan Sand. EOR field tests recently begun in the northeastern part of the oil field require detailed characterization of multiple sandstone reservoirs and mudstone seals. CO2 will be injected into the Upper Donovan 14-1 and 16-2 sandstone units in the Citronelle Unit B-19-10 #2 well (Alabama State Oil and Gas Board Permit No. 3232). Detailed characterization of reservoirs and seals helps guide the EOR project. Precise targeting of CO2 injection sites and expectations of oil volumes to be produced from nearby wells are informed by 3D geologic models that are based on stratigraphic correlation and core description. Stratigraphic cross sections correlating sandstone units in the Upper Donovan demonstrate extreme facies heterogeneity. Of particular interest is the 16-2 sand, which is interpreted as a composite of two tiers of channel fills. Pay strata are typically developed in the lower tier, where CO2 is being injected. The upper tier is highly heterogeneous and is interpreted to contain sandstone fills of variable reservoir quality, as well as mudstone plugs. The Donovan Sand was deposited in a coastal setting in which a shifting mosaic of terrestrial, marginal marine, and nearshore environments was controlled by subsidence, eustasy, and varying rates of sediment input. Lithofacies include, in order of decreasing abundance, silty sandstone and sandy siltstone, mudstone, vertic paleosols, very fine to fine sandstone, shale-pebble conglomerate, and oyster biostromes. Most reservoirs in Citronelle Field consist of very fine to fine-grained arkosic sandstone. The pore system in these strata consists of primary interparticle pores and secondary pores generated by feldspar diagenesis. Many pores are completely or partly occluded by clay matrix, clay cement, syntaxial quartz overgrowths, dolomite cement, and pyrobitumen.